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                <h1 id="哨兵集群：哨兵挂了，主从还能切换吗"><a href="#哨兵集群：哨兵挂了，主从还能切换吗" class="headerlink" title="哨兵集群：哨兵挂了，主从还能切换吗"></a>哨兵集群：哨兵挂了，主从还能切换吗</h1><h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>我们之前在<a target="_blank" rel="noopener" href="https://kuangtf.gitee.io/2021/11/04/redis/huan-bu-hui-shao-bing-ji-zhi-wo-jiao-ni-a/">哨兵机制</a>中讲过了（如果没有看过这篇文章的，必须先看这篇文章之后再来看本篇），哨兵可以实现主从库的自动切换。通过部署多个实例，就形成了一个哨兵集群，哨兵集群中的多个实例共同判断，可以降低对主库下线的误判率。</p>
<p>但是你有没有想过这样一个问题：如果有哨兵实例在运行时发生了故障，主库还能正常切换吗？</p>
<p>实际上，一旦多个实例组成了哨兵集群，即使有哨兵实例出现故障挂掉了，其他哨兵还继续协作完成主从库切换的工作，包括判定主库是不是处于下线状态，选择新主库，以及通知从库和客户端。</p>
<p>如果你部署过哨兵集群的话就会知道，在配置哨兵的信息时，我们只需要用到下面的这个配置项，设置主库的 IP和端口，并没有配置其他哨兵的连接信息。  </p>
<pre class="line-numbers language-none"><code class="language-none">sentinel monitor &lt;master-name&gt; &lt;ip&gt; &lt;redis-port&gt; &lt;quorum&gt;<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>这些哨兵实例既然都不知道彼此的地址，又是怎么组成集群的呢？要弄明白这个问题，我们就需要学习一下哨兵集群的组成和运行机制了。  </p>
<h2 id="基于-pub-sub-机制的哨兵集群组成"><a href="#基于-pub-sub-机制的哨兵集群组成" class="headerlink" title="基于 pub/sub 机制的哨兵集群组成"></a>基于 pub/sub 机制的哨兵集群组成</h2><p>哨兵实例之间可以相互发现，要归功于 Redis 提供的 pub/sub 机制，也就是发布 / 订阅机制。  </p>
<p>哨兵只要和主库建立起了连接，就可以在主库上发布消息了，比如说发布它自己的连接信息（IP 和端口）。同时，它也可以从主库上订阅消息，获得其他哨兵发布的连接信息。当多个哨兵实例都在主库上做了发布和订阅操作后，它们之间就能知道彼此的 IP 地址和端口。</p>
<p>除了哨兵实例，我们自己编写的应用程序也可以通过 Redis 进行消息的发布和订阅。所以，为了区分不同应用的消息，Redis 会以频道的形式，对这些消息进行分门别类的管理。所谓的频道，实际上就是消息的类别。当消息类别相同时，它们就属于同一个频道。反之，就属于不同的频道。只有订阅了同一个频道的应用，才能通过发布的消息进行信息交换。  </p>
<p>例如：在主从集群中，主库上有一个名为“_ _ sentinel _ _:hello” 的频道，不同哨兵就是通过它来相互发现，实现互相通信的。在下图中，哨兵 1 把自己的 IP（172.16.19.3）和端口（26579）发布到“ _ _sentinel _ _:hello”频道上，哨兵 2 和 3 订阅了该频道。那么此时，哨兵 2 和 3 就可以从这个频道直接获取哨兵 1 的 IP 地址和端口号。  </p>
<p>然后，哨兵 2、3 可以和哨兵 1 建立网络连接。通过这个方式，哨兵 2 和 3 也可以建立网络连接，这样一来，哨兵集群就形成了。它们相互间可以通过网络连接进行通信，比如说对主库有没有下线这件事儿进行判断和协商。</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327120438751.png" alt="image-20220327120438751" style="zoom:80%;" />

<p>哨兵除了彼此之间建立起连接形成集群外，还需要和从库建立连接。这是因为，在哨兵的监控任务中，它需要对主从库都进行心跳判断，而且在主从库切换完成后，它还需要通知从库，让它们和新主库进行同步。  </p>
<p>那么，哨兵是如何知道从库的 IP 地址和端口的呢？  </p>
<p>这是由哨兵向主库发送 INFO 命令来完成的。就像下图所示，哨兵 2 给主库发送 INFO 命令，主库接受到这个命令后，就会把从库列表返回给哨兵。接着，哨兵就可以根据从库列表中的连接信息，和每个从库建立连接，并在这个连接上持续地对从库进行监控。哨兵 1和 3 可以通过相同的方法和从库建立连接。</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327120445902.png" alt="image-20220327120445902" style="zoom:80%;" />

<p>你看，通过 pub/sub 机制，哨兵之间可以组成集群，同时，哨兵又通过 INFO 命令，获得了从库连接信息，也能和从库建立连接，并进行监控了。但是，哨兵不能只和主、从库连接。因为，主从库切换后，客户端也需要知道新主库的连接信息，才能向新主库发送请求操作。所以，哨兵还需要完成把新主库的信息告诉客户端这个任务。</p>
<p>而且，在实际使用哨兵时，我们有时会遇到这样的问题：如何在客户端通过监控了解哨兵进行主从切换的过程呢？比如说，主从切换进行到哪一步了？这其实就是要求，客户端能够获取到哨兵集群在监控、选主、切换这个过程中发生的各种事件。</p>
<p>此时，我们仍然可以依赖 pub/sub 机制，来帮助我们完成哨兵和客户端间的信息同步。</p>
<h2 id="基于-pub-sub-机制的客户端事件通知"><a href="#基于-pub-sub-机制的客户端事件通知" class="headerlink" title="基于 pub/sub 机制的客户端事件通知"></a>基于 pub/sub 机制的客户端事件通知</h2><p>从本质上说，哨兵就是一个运行在特定模式下的 Redis 实例，只不过它并不服务请求操作，只是完成监控、选主和通知的任务。所以，每个哨兵实例也提供 pub/sub 机制，客户端可以从哨兵订阅消息。哨兵提供的消息订阅频道有很多，不同频道包含了主从库切换过程中的不同关键事件。</p>
<p>频道有这么多，一下子全部学习容易丢失重点。为了减轻你的学习压力，我把重要的频道汇总在了一起，涉及几个关键事件，包括主库下线判断、新主库选定、从库重新配置。  </p>
<table>
<thead>
<tr>
<th>事件</th>
<th>相关频道</th>
</tr>
</thead>
<tbody><tr>
<td>主库下线事件</td>
<td>+sdown（实例进入“主观下线”状态）</td>
</tr>
<tr>
<td></td>
<td>-sdown（实例退出“主观下线”状态）</td>
</tr>
<tr>
<td></td>
<td>+odown（实例进入“客观下线”状态）</td>
</tr>
<tr>
<td></td>
<td>-odown（实例退出“客观下线“状态）</td>
</tr>
<tr>
<td>从库重新配置事件</td>
<td>+ slave–reconf–sent（哨兵发送SLAVEOF命令重新配置从库）</td>
</tr>
<tr>
<td></td>
<td>+slave–reconf–inprog（从库配置了新主库，但尚未进行同步）</td>
</tr>
<tr>
<td></td>
<td>+slave–reconf–done（从库配置了新主库，且和新主库完成同步）</td>
</tr>
<tr>
<td>新主库切换</td>
<td>+switch–master（主库地址发生变化）</td>
</tr>
</tbody></table>
<p>知道了这些频道之后，你就可以让客户端从哨兵这里订阅消息了。具体的操作步骤是，客户端读取哨兵的配置文件后，可以获得哨兵的地址和端口，和哨兵建立网络连接。然后，我们可以在客户端执行订阅命令，来获取不同的事件消息。  </p>
<p>举个例子，你可以执行如下命令，来订阅“所有实例进入客观下线状态的事件”：  </p>
<pre class="line-numbers language-none"><code class="language-none">SUBSCRIBE +odown<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>当然，你也可以执行如下命令，订阅所有的事件：  </p>
<pre class="line-numbers language-none"><code class="language-none">PSUBSCRIBE *<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>当哨兵把新主库选择出来后，客户端就会看到下面的 switch-master 事件。这个事件表示主库已经切换了，新主库的 IP 地址和端口信息已经有了。这个时候，客户端就可以用这里面的新主库地址和端口进行通信了。  </p>
<pre class="line-numbers language-none"><code class="language-none">switch-master &lt;master name&gt; &lt;oldip&gt; &lt;oldport&gt; &lt;newip&gt; &lt;newport&gt;<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>有了这些事件通知，客户端不仅可以在主从切换后得到新主库的连接信息，还可以监控到主从库切换过程中发生的各个重要事件。这样，客户端就可以知道主从切换进行到哪一步了，有助于了解切换进度。</p>
<p>好了，有了 pub/sub 机制，哨兵和哨兵之间、哨兵和从库之间、哨兵和客户端之间就都能建立起连接了，再加上我们之前介绍主库下线判断和选主依据，哨兵集群的监控、选主和通知三个任务就基本可以正常工作了。不过，我们还需要考虑一个问题：主库故障以后，哨兵集群有多个实例，那怎么确定由哪个哨兵来进行实际的主从切换呢？  </p>
<h2 id="由哪个哨兵执行主从切换？"><a href="#由哪个哨兵执行主从切换？" class="headerlink" title="由哪个哨兵执行主从切换？"></a>由哪个哨兵执行主从切换？</h2><p>确定由哪个哨兵执行主从切换的过程，和主库“客观下线”的判断过程类似，也是一个“投票仲裁”的过程。任何一个实例只要自身判断主库“主观下线”后，就会给其他实例发送 is-master-downby-addr 命令。接着，其他实例会根据自己和主库的连接情况，做出 Y 或 N 的响应，Y 相当于赞成票，N 相当于反对票。</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327120500404.png" alt="image-20220327120500404" style="zoom:80%;" />

<p>一个哨兵获得了仲裁所需的赞成票数后，就可以标记主库为“客观下线”。这个所需的赞成票数是通过哨兵配置文件中的 quorum 配置项设定的。例如，现在有 5 个哨兵，quorum 配置的是 3，那么，一个哨兵需要 3 张赞成票，就可以标记主库为“客观下线”了。这 3 张赞成票包括哨兵自己的一张赞成票和另外两个哨兵的赞成票。  </p>
<p>此时，这个哨兵就可以再给其他哨兵发送命令，表明希望由自己来执行主从切换，并让所有其他哨兵进行投票。这个投票过程称为“Leader 选举”。因为最终执行主从切换的哨兵称为 Leader，投票过程就是确定 Leader。  </p>
<blockquote>
<p>每个哨兵只能投一张赞成票。</p>
</blockquote>
<p>在投票过程中，任何一个想成为 Leader 的哨兵，要满足两个条件：第一，拿到半数以上的赞成票；第二，拿到的票数同时还需要大于等于哨兵配置文件中的 quorum 值。以 3 个哨兵为例，假设此时的 quorum 设置为 2，那么，任何一个想成为 Leader 的哨兵只要拿到2 张赞成票，就可以了。</p>
<p>如果有多个最多票的哨兵，或者没有达到 quorum 值，就会等待一段时间（也就是哨兵故障转移超时时间的 2 倍），再重新选举。  </p>
<p>这是因为，哨兵集群能够进行成功投票，很大程度上依赖于选举命令的正常网络传播。如果网络压力较大或有短时堵塞，就可能导致没有一个哨兵能拿到半数以上的赞成票。所以，等到网络拥塞好转之后，再进行投票选举，成功的概率就会增加。  </p>
<p>投票的依据就是：哨兵如果没有给自己投票，就会把票投给第一个给它发送投票请求的哨兵。后续再有投票请求来，哨兵就拒接投票了。</p>
<p>好啦，到这里我们就讲解完了哨兵集群，你知道你搞没搞懂呢？</p>
<blockquote>
<p>巨人的肩膀：</p>
<p>极客时间 Redis 核心原理与实战</p>
</blockquote>

                
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                        var startYear = "2019";
                        var startMonth = "6";
                        var startDate = "28";
                        var startHour = "0";
                        var startMinute = "0";
                        var startSecond = "0";
                        var todayYear = today.getFullYear();
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                        var todayHour = today.getHours();
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                        var t1 = Date.UTC(startYear, startMonth, startDate, startHour, startMinute, startSecond);
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                        var diff = t2 - t1;
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                            var yearsAndDaysTip = 'This site has been running for ' + diffYears + ' years and '
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                            if (language === 'zh-CN') {
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                            } else if (language === 'zh-HK') {
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